White roofs – an effective cooler for houses

Another option is not to whitewash roofs, but to green them with foliage. This is already being adopted in many cities

There is a third option competing for roof space to take the heat out of cities — covering them in photovoltaic cells. PV cells are dark, and so do not reflect much solar radiation into space. But that is because their business is to capture that energy and convert it into low-carbon electricity.

Solar panels “cool daytime temperatures in a way similar to increasing albedo via white roofs,” according to a study by scientists at the University of New South Wales. The research, published in the journal Scientific Reports last year, found that in a city like Sydney, Australia, a city-wide array of solar panels could reduce summer maximum temperatures by up to 1 degree C. …….

The lesson then is that light, reflective surfaces can have a dramatic impact in cooling the surrounding air – in cities, but in the countryside too. Whitewashed walls, arrays of photovoltaic cells, and stubble-filled fields can all provide local relief during the sweltering decades ahead.

Urban Heat: Can White Roofs Help Cool World’s Warming Cities?

It has long been known that installing white roofs helps reduce heat buildup in cities. But new research indicates that making surfaces more light-reflecting can have a significant impact on lowering extreme temperatures – not just in cities, but in rural areas as well.Yale Environment 360 BY FRED PEARCE • MARCH 7, 2018 Summers in the city can be extremely hot — several degrees hotter than in the surrounding countryside. But recent research indicates that it may not have to be that way. The systematic replacement of dark surfaces with white could lower heat wave maximum temperatures by 2 degrees Celsius or more. And with climate change and continued urbanization set to intensify “urban heat islands,” the case for such aggressive local geoengineering to maintain our cool grows.The meteorological phenomenon of the urban heat island has been well known since giant cities began to emerge in the 19th century. The materials that comprise most city buildings and roads reflect much less solar radiation – and absorb more – than the vegetation they have replaced. They radiate some of that energy in the form of heat into the surrounding air.

The darker the surface, the more the heating. Fresh asphalt reflects only 4 percent of sunlight compared to as much as 25 percent for natural grassland and up to 90 percent for a white surface such as fresh snow.

Most of the roughly 2 percent of the earth’s land surface covered in urban development suffers from some level of urban heating. New York City averages 1-3 degrees C warmer than the surrounding countryside, according to the U.S. Environmental Protection Agency – and as much as 12 degrees warmer during some evenings. The effect is so pervasive that some climate skeptics have seriously claimed that global warming is merely an illusion created by thousands of once-rural meteorological stations becoming surrounded by urban development.

Climate change researchers adjust for such measurement bias, so that claim does not stand up. Nonetheless, the effect is real and pervasive. So, argues a recent study published in the journal Nature Geoscience, if dark heat-absorbing surfaces are warming our cities, why not negate the effect by installing white roofs and other light-colored surfaces to reflect back the sun’s rays?

During summer heat waves, when the sun beats down from unclouded skies, the creation of lighter land surfaces “could help to lower extreme temperatures… by up to 2 or 3 degrees Celsius” in much of Europe, North America, and Asia, says Sonia Seneviratne, who studies land-climate dynamics at the Swiss Federal Institute of Technology (ETH) in Zurich, and is co-author of the new study. It could save lives, she argues, and the hotter it becomes, the stronger the effect.

Seneviratne is not alone in making the case for boosting reflectivity. There are many small-scale initiatives in cities to make roof surfaces more reflective. New York, for instance, introduced rules on white roofs into its building codes as long ago as 2012. Volunteers have taken white paint to nearly 7 million square feet of tar roofs in the city, though that is still only about 1 percent of the potential roof area.

Chicago is trying something similar, and last year Los Angeles began a program to paint asphalt road surfaces with light gray paint. Outside the United States, cool-roof initiatives in cities such as Melbourne, Australia are largely limited to encouraging owners to cool individual buildings for the benefit of their occupants, rather than trying to cool cities or neighborhoods.

The evidence of such small-scale programs remains anecdotal. But now studies around the world are accumulating evidence that the benefits of turning those 1 percents into 100 percents could be transformative and could save many lives every year.

Keith Oleson of the National Center for Atmospheric Research in Boulder, Colorado looked at what might happen if every roof in large cities around the world were painted white, raising their reflectivity — known to climate scientists as albedo — from a typical 32 percent today to 90 percent. He found that it would decrease the urban heat island effect by a third — enough to reduce the maximum daytime temperatures by an average of 0.6 degrees C, and more in hot sunny regions such as the Arabian Peninsula and Brazil.

Other studies suggest even greater benefits in the U.S. In a 2014 paper, Matei Georgescu of Arizona State University found that “cool roofs” could cut temperatures by up to 1.5 degrees C in California and 1.8 degrees in cities such as Washington, D.C.

But it may not just be urban areas that could benefit from a whitewashing. Seneviratne and her team proposed that farmers could cool rural areas, too, by altering farming methods. Different methods might work in different regions with different farming systems. And while the percentage changes in reflectivity that are possible might be less than in urban settings, if applied over large areas, she argues that they could have significant effects……..

Another option is not to whitewash roofs, but to green them with foliage. This is already being adopted in many cities. In 2016, San Francisco became the first American city to make green roofs compulsory on some new buildings. New York last year announced a $100-million program for cooling neighborhoods with trees. So which is better, a white roof or a “green” roof?

Evidence here is fragmentary. But Georgescu found a bigger direct cooling effect from white roofs. Vincenzo Costanzo, now of the University of Reading in England, has reached a similar conclusion for Italian cities. But green roofs may have other benefits. A study in Adelaide, Australia, found that besides delivering cooling in summer, they also act as an insulating layer to keep buildings warmer in winter.